III-V nitride compound semiconductors are excellent candidates as useful materials for short-wavelength light emitting devices because of their wide band gap. Among these, extensive research has been conducted relating to gallium nitride-based compound semiconductors (GaN based semiconductors: AlGaInN). As a result, blue light emitting diodes (LED) and green LEDs have already been put to practical use. Furthermore, in order to increase the storage capacity of an optical disk apparatus, a semiconductor laser with its oscillation wavelength in the 400-nm band is in strong demand. For this reason, semiconductor lasers using GaN based semiconductors have attracted widespread attention, and are now approaching a level of practically use.
Because it is difficult to form bulk single crystals with a large area for GaN-based semiconductors, the crystals are generally grown on a sapphire or SiC substrate. However, since there are lattices mismatches between GaN and both of these substrates, it is difficult to achieve coherent crystal growth. Therefore, in the thus obtained GaN-based semiconductor layers, a great number of dislocations (edge dislocations, screw dislocations, mixed dislocations) exist, which significantly decrease the reliability of the semiconductor laser. For example, when a GaN-based semiconductor crystal is grown on a sapphire substrate, approximately 1×109 cm−2 dislocations exist in the semiconductor layer.
A method that employs a selective lateral growth technique is thus proposed for decreasing the number of dislocations. This method is effectively used in systems having a large lattice mismatch and can reduce threading dislocations. Hereunder, a method for forming a GaN-based semiconductor layer by the ELOG (Epitaxial Lateral Overgrowth) technique, which is one of the approaches employing the selective lateral growth technique, will be explained. FIG. 6 shows a method for forming a substrate having a reduced number of dislocations in which the ELOG technique is employed.
First, employing MOVPE or a like crystal growing method, the first GaN layer 202 is formed on a sapphire substrate 201 (FIG. 6(a)).
Then, an SiO2 layer is deposited by a plasma CVD method or the like and the SiO2 layer is processed by photolithography and etching to form a mask layer 203 made of SiO2 film. This mask layer 203 is composed of a plurality of band-like portions 203a that have a width of approximately 12 lm and are arranged with intervals therebetween of approximately 3 im. The portions of the first GaN layer 202 that are exposed by the intervals between the band-like portions 203a become seed crystal portions 202a on which a second GaN layer 204, which is described later, is deposited (FIG. 6(b)).
Then, a GaN layer is selectively grown from the seed crystal portions 202a. Here, rather than directly depositing the GaN layer on the mask layer 203, the GaN layer that is deposited on the seed crystal portions 202a laterally grows so as to cover the mask layer 203 (FIG. 6(c)).
Thereby, the mask layer 203 is entirely covered with the GaN layer, covering the entire surface of the substrate with the second GaN layer 204 (FIG. 6(d)).
In the thus structured substrate, the dislocation density of the part of the second GaN layer 204 deposited on the seed crystal portions 202a is high, at approximately 1×109 cm−2, while the dislocation density of the part of the second GaN layer 204 deposited by lateral growth on the mask layer 203 is low, at approximately 1×107 cm−2. Therefore, for example, when a semiconductor laser is fabricated, a current injection region is formed directly above the low-dislocation-density mask layer 203, enhancing the reliability of the semiconductor laser.
However, in the thus formed substrate, the band-like portions 203a of the mask layer 203 and the seed crystal portions 202a are alternately formed in a regular pattern. This makes it difficult to distinguish between the regions with high and low dislocation density, when forming the above-mentioned current injection region, which in turn makes it difficult to select the appropriate location for forming the current injection region. For this reason, in heretofore-used methods, a reference point called an alignment mark was formed on a layer other than the second GaN layer 204, and the location for forming the current injection region was selected by referring to the alignment mark.
However, in the above case, the additional process for forming the alignment mark makes the fabrication process complicated and the fabrication time longer.
The present invention aims to solve the above problems with a method for fabricating a GaN-based semiconductor laser device in which the current injection region can be positioned while using the heretofore-employed fabricating process without an additional process, and to provide a semiconductor substrate for use therein.